For example, one of prior art plasma-arc cutters with a plasma-arc power supply apparatus generates a plasma-arc between a main electrode and a workpiece by utilizing a pilot arc generated between the main electrode and a nozzle electrode. The main and nozzle electrodes are disposed on a plasma-torch. For example, Japanese Examined Patent Publication No. HEI 6-75791 describes a technique for stabilizing a pilot arc and a plasma-arc. According to the technique disclosed in this Japanese patent publication, a first output terminal of a DC power supply device which converts AC power into DC power is connected to a nozzle electrode on a plasma torch through a series circuit including arc current detecting means, a current limiting resistor and a switch, while a second output terminal of the DC power supply device Is connected through a high frequency generator to a main electrode on the plasma torch. The high frequency generator is used to apply a high frequency voltage between the main and nozzle electrodes for generating a pilot arc therebetween. These components form a pilot arc generating circuit for generating a pilot arc between the main and nozzle electrodes. DC current flowing through the pilot arc generating circuit, i.e. a pilot current Ip is detected by the arc current detecting means.
The first output terminal of the DC power supply device is also connected to a workpiece through the arc current detecting means and cutting current detecting means, which form a plasma-arc generating circuit for generating a plasma-arc between the main electrode and the workpiece. DC current flowing in the plasma-arc generating circuit, i.e. a plasma-arc current Ic is detected by the arc current detecting means and the cutting current detecting means.
A detection signal from the arc current detecting means which represents the detected pilot current Ip or plasma current Ic is applied to a constant current control circuit. The constant current control circuit receives a reference value from reference value setting means. The constant current control circuit controls the DC power supply device in such a manner as to make the detection signal from the ac current detecting means equal to the reference value. The reference value set by the reference value setting means is switched to a reference value for use in controlling the plasma current Ic when the level of the detection signal from the cutting current detecting means reaches a predetermined level.
In operation of the above-described plasma cutter, when the switch is closed and the high frequency generator is activated, a pilot arc is generated between the main and nozzle electrodes so that a pilot current Ip flows. In this stage, no plasma arc has been generated yet between th main electrode and the workpiece, and the current Ia flowing through the arc current detecting means is only the pilot current Ip. Thus, the arc current detecting means detects only the pilot current Ip. The constant current control circuit controls the DC power supply device in such a manner that the pilot current Ip detected by the arc current detecting means is at a constant value as determined by the reference value set by the reference value setting means.
By bringing the plasma torch closer to the workpiece while the pilot arc is present, a plasma-arc is generated between the main electrode and the workpiece so that the plasma current Ic flows. Then, the switch is opened to cause the pilot arc to cease so that the current Ia flowing through the arc current detecting means is provided by the plasma current Ic only. At the same time, the level of the detection-signal of the plasma current detecting means reaches the predetermined level. Accordingly, th reference value set by the reference value setting means is switched to the reference value for use in controlling the plasma current Ic. Then, the constant current control circuit controls the DC power supply device in such a manner that the plasma current Ic is maintained at a constant value as determined by the reference value for controlling the plasma current Ic.
According to the above-described prior art apparatus, the plasma current Ic flows in both of the arc current detecting means and the cutting current detecting means. Generally, the plasma current Ic is significantly larger than the pilot current Ic, and is a large current o for example, several hundred amperes. In order to detect such a large current Ic, the detecting means must have large current capacity. Then, the above-described prior art apparatus requires two of such large capacity current detecting means, which, in turn, disadvantageously causes the size of the plasma-arc cutter to become larger.
An object of the present invention, therefore, is to provide a plasma-arc power supply apparatus which can be of a size smaller than that of prior art apparatus, by using current detecting means having current capacities appropriate for detecting the pilot current Ip and the plasma current Ic, respectively, so that the number of large current capacity detecting means as required in the above-described prior art apparatus can be reduced to one so that the plasma-arc cutter as a whole can be fabricated smaller. Another object of the present invention is to provide a plasma-arc power supply apparatus in which both of the pilot current Ip and the plasma current Ic can be detected by a single current detecting means.